This EternaBond and Karnak #19 Ultra, do they sell them at the Roof Center or do you have to order online ?

If the 'Roof Center' is a roofer's supply house, I'd think so. If not, Google Bradco, Allied Builders Supply, ABC Supply of the like. One of them will have it.
I've never shopped for professional stuff at HD, or the other one, so I have no idea what they stock.

i trimmed a perimeter around the hole (cca 3" give or take on all sides), and cleaned it all down to the wood.

then i made a 3/4" OSB adapter lid on top of which i set the curb. (the metal brackets from the picture are just temporary, will screw it from the plywood into the 2x6).

this does leave some marginal gap between the new OSB and the old mod bit roofing, which i plan to temporarily patch with EternaBond.

i am posting this to get a confirmation from you that it is all done right and that torched tar, as the permanent solution, will be able to seal the gap and keep it water tight. if i need to do something else, like e.g. put some sheet metal in between the old roof sheathing 1x lumber and the OSB lid or something like that, please let me know.

1/4" is too much of a gap for mod-bit. It needs to be tight. It would have been better to just reinstall the cut out pieces of mod-bit instead of using OSB to raise the level to match the existing. Am I missing something?

It would have been better to just reinstall the cut out pieces of mod-bit instead of using OSB to raise the level to match the existing. Am I missing something?

it needed a panel adapter, which is what the OSB does. "the cut out pieces of mod-bit" were like a mess, like 100s of little geometrically irregular shredded or chopped pieces.

can i fill the gap with something else before pouring the new tar? the lines of the cut out mod-bit were irregular, naturally, and the panel had to be an approximation, i.e. had to have some sort of spacing on all sides to make sure it fits.

i could do that, however, can't i just stuff something in the gap, like the foamy backer rod by Sika before pouring the new tar.

man, i really believe in providing for a margin of error, especially when you have to cut something to fit somewhere whose shape is not exactly geometrically straight. every time i try to make something fit exact, i have to go back and forth and cut it more, trim it, plane it, sand it, jigsaw it and it is frikkin annoying. so i'd rather make it a little smaller and then fill the gaps.

which explains why i HATE cutting subflooring panels and, to a minor degree, drywall (cause it is easier to cut and adjust).